1. Field of the Invention
The present invention relates to an external magnetic field generating device for an optical magnetic disk apparatus which is suitably used when information is recorded on or erased from an optical magnetic disk by means of a laser beam.
2. Related Background Art
When magneto-optic information is recorded on an optical magnetic disk by means of a laser beam, a laser beam digitally modulated by an information signal is irradiated onto a film magnetized in one vertical direction beforehand to heat the vertically magnetized film to a temperature higher than the Curie point while a direct-current bias direction of a magnetic field which is opposite to that in which the film is magnetized is applied to the film by an external magnetic field generating device. When cooled, the film is magnetized in a direction opposite to that in which the surrounding portion is magnetized, whereby a row of recording bits is formed on the magnetic disk in accordance with the information. When the recording bits are erased, a non-modulated laser beam is irradiated on the recording bits to heat the bits to a temperature higher than the Curie point, and the vertically magnetized film is magnetized again by the direct-current bias in the same direction as that in which the surrounding portion is magnetized when it is cooled.
More particularly, information is recorded on and erased from the optical magnetic disk by heating the vertically magnetized film to a temperature higher than the Curie point and thereby demagnetizing the portion of the film irradiated by the laser beam and then by magnetizing that portion in the same direction as that of the direct-current bias, e.g., in the direction opposite to that in which the surrounding portion is magnetized. The direction of the direct-current bias applied to the magnetized film during recording is opposite to that which is applied during erasure.
While information is recorded on or erased from the optical magnetic disk, the laser beam gradually moves sequentially or stepwise in the radial direction of the disk which is rotated at a fixed speed. In consequence, the amount of light energy per unit time absorbed by the outer peripheral portion of the magnetized film differs from that absorbed by the inner peripheral portion thereof, making uniform and appropriate recording and erasure impossible.
Accordingly, it has been proposed to vary the intensity of the laser beam in accordance with the irradiating position, i.e., to gradually increase the intensity of the laser beam as the laser beam approaches the outer periphery of the optical magnetic disk, while the intensity of the external magnetic field remains constant. It has also been disclosed (in, for example, Japanese Patent Appln. Laid-Open No. 60-147948) to change the intensity of the external magnetic field in accordance with the laser beam irradiating position while the intensity of the laser beam remains constant. In the former technique it is difficult to maintain the intensity of the laser beam constant, and a complicated control circuit is necessary. Therefore, the latter technique is generally adopted.
FIG. 7 shows a conventional example of an external magnetic field generating device employed in the latter technique. An external magnetic field generating device 61 includes a main yoke 62 and an exciting coil 63 wound around the main yoke 62. The external magnetic field generating device 61 is disposed close to and in opposed relation to the surface of an optical magnetic disk 4 which is opposite to that irradiated by a laser beam 5 in such a manner that the longitudinal direction of the main yoke 62 coincides with the radial direction of the optical magnetic disk 4. Also, the external magnetic field generating device 61 is inclined by an angle .theta., i.e., a gap between an outer end "a" of the main yoke 62 and the surface of the disk is smaller than that between an inner end "b" of the main yoke 62 and the disk surface, such that the intensity of the magnetic field generated by the external magnetic field generating device 61 gradually increases as the magnetic field approaches the outer periphery of the optical magnetic disk 4, as shown by a solid line 66 in FIG. 8.
In a case where the former technique in which the intensity of laser beam 5 is changed is adopted, the external magnetic field generating device 61 is disposed parallel to the surface of the disk to obtain a magnetic field intensity indicated by a broken line 67 in FIG. 8.
However, since the above-described conventional external magnetic field generating device must be provided in an inclined state relative to the optical magnetic disk, mounting and adjustment thereof are a troublesome task as compared with the case in which the external magnetic field generating device is disposed parallel. Furthermore, the portion of the external magnetic field generating device located close to the outer periphery of the disk must be closer to the disk surface than the portion thereof located near the center of the disk. A gap from 1 to 1.5 mm is required in minimum between the disk surface of the external magnetic field generating device even when a flat magnetic field is obtained. Thus, there is a limitation to the distance between the disk surface and the external magnetic field generating device from the viewpoint of vibrations of the disk surface.
Paying attention to the intensity of the external magnetic field, since the portions of the magnetic field intensity corresponding to the two end portions "a" and "b" are excessively higher than the central flat portion thereof due to the edge effect, as shown in FIG. 8, it is impossible to obtain a distribution of the magnetic field intensity which is linearly inclined over the entire recording area A of the optical magnetic disk 4. Attempts have also been made to use only a central linearly inclined magnetic field intensity area B. However, this requires a long external magnetic field generating device, as shown in FIG. 9. Also, the external magnetic field generating device must be constructed such that the magnetic field intensity portions C caused by the edge effect correspond to the portion of the disk located outside of the recording area thereof, as shown in FIG. 10. This increases the size of the device.
Furthermore, to prevent generation of the edge effect, it has conventionally been proposed to obtain a uniform magnetic field by inclining the inner and outer end portions of the surface of the main yoke which is opposed to the disk (Japanese Patent Appln. Laid-Open No. 62-65202). However, this requires that the external magnetic field generating device be disposed inclined relative to the disk surface, and thus makes mounting difficult. Also, the conventional edge effect generation prevention method is not effective because it wastes a strong magnetic field.